A "converter circuit" is defined as a circuit which is part of a device for converting high voltage, which converts direct voltage to alternating voltage or the converse, or converts direct voltage to direct voltage upward or downward with respect to the input voltage level or alternating voltage to alternating voltage upwardly or downwardly from one frequency to another.
Such circuits may, for example, be used in voltage source converters for transmission of electric power through High Voltage Direct Current (HVDC) for conversion of direct voltage to alternating voltage and conversely. Plants for reactive power compensation (RPC) may also utilize such converters. These converters may in such plants typically have to maintain voltages within the range of 10-500 kV, although other voltages are conceivable. This makes it necessary to connect many power semiconductor devices in series to distribute the voltage among them, since they normally each may only hold 1-5 kV.
Examples of such power semiconductor devices of turn-off type are gate turn-off thyristors (GTO), MOSFETs and IGBTs (Insulated Gate Bipolar Transistors), of which the latter are preferable since they combine good power handling ability with features making them suited for connection in series in so-called IGBT valves in converters, since they may easily be turned on and turned off simultaneously.
When using many power semiconductor devices connected in series in a high voltage converter circuit of the type defined in the introduction, there is a problem in supplying the individual drive units with energy. A possible solution is to provide the different drive units with energy from, for example, ground potential while utilizing individual transformers, one for each drive unit. This solution is very expensive, especially in high voltage converter circuits in which the potential differences between ground and the different drive units is high, such as, for example, tens of kV or more.